1. Field of the Invention
The present invention relates to lithography, more particularly, to providing systems and methods for advanced lithographic reticle inspection.
2. Related Art
Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays (e.g., liquid crystal displays), semiconductor wafers, circuit boards, various integrated circuits, print heads, macro/nano-fluidic substrates, and the like. During lithography, a substrate, which is disposed on a substrate stage, is exposed to an image projected onto the surface of the substrate by exposure optics through the use of a patterning device, such as a reticle, located within a lithography apparatus.
The projected image produces changes in the characteristics of a layer, for example, photoresist, deposited on the surface of the substrate. These changes correspond to the features projected onto the substrate during exposure. Subsequent to exposure, the layer can be etched or otherwise processed to produce a patterned layer. The pattern corresponds to those features projected onto the substrate during exposure. The patterned layer is then used to remove or further process exposed portions of underlying structural layers within the substrate, such as conductive, semiconductive, or insulative layers. This process is repeated, together with other steps, until the desired features have been formed on the surface, or in various layers, of the substrate.
Reticles, which can also be referred to as masks, are created in mask houses from high quality blanks. They are inspected for pattern accuracy and defects. Once deemed good they are put into service where they are typically inspected again to ensure that they are free of particles. Nonetheless, substrate errors occur due to patterning errors attributable to reticles. In particular, reticles are being distorted out of their ideal shape, which leads to inaccurate pattern transfer.
Reticles can be distorted, for example, as they are mounted onto a reticle stage. The distortion manifests itself as an error in printing on a wafer either due to focus, telecentricity, pattern distortion or induced retardation. As a result, reticles can have varying stress birefringence and thereby impact printing on the wafer, especially on machines where the illumination is polarized. These reticles will nominally have acceptable levels of birefringence when made, but may not stay in an acceptable state on the reticle stage. The problem is exacerbated by the need to clamp the reticle very firmly to the stage to avoid slipping during high G acceleration in newer and increasingly faster lithographic machines. Additionally, during the loading process a particle could get trapped between a reticle and a reticle chuck, which could also lead to pattern distortions caused by the reticle.
What are needed are systems and in situ methods for advanced reticle inspection that can identify unwanted distortions in the reticles when they are mounted onto a reticle stage.